1. Technical Field
This invention relates to a technology capable of increasing a frequency utilization efficiency by an adaptive modulation communication based upon a feedback of a downlink communication quality in a base station equipped with a smart antenna technology.
2. Background Art
Recently, smart antenna technology using array antennas has been practically used for base stations and access points (hereinafter, collectively referred to as base station) of wireless communication systems such as portable telephones and wireless LANs. The operating principle of the smart antenna technology is disclosed in, for instance, B. Widrow, et al.: “Adaptive Antenna Systems”, Proceedings IEEE, Vol. 55, No. 12, pages 2143 to 2159, December 1967. Base stations in which a plurality of terminals commonly utilize the same time and the same frequency and spatial multiplexing communications are performed based upon the smart antenna technology is disclosed in, for example, JP 3167682 B.
Further, while needs for data communications in wireless communication systems are increased, the cdma2000 EV-DO (Evolution Data Only) system having purposes of increasing peak transmission speeds of downlinks and of realizing high throughputs has been standardized as a packet transmission system of IMT-2000 (for example, 3GPP2 C.S0024-A “cdma2000 High Rate Packet Data Air Interface Specification”, pages 13-42 to 13-78, Mar. 31, 2004). In the above-described high rate packet communication system, the packet scheduling is carried out in order to effectively utilize limited frequencies and limited time.
The packet scheduling technique in the EV-DO system is such a technique that a base station selects a mobile station which becomes a transmission destination of downlink data every time a time slot of 1.67 milliseconds has elapsed. In the case where mobile stations are selected, a wireless communication apparatus having high priority data and another wireless communication apparatus whose downlink communication quality is high are selected with a top priority. In this case, a downlink indicates a direction of a wireless communication that the base station transmits and the mobile station receives. In “Data Throughput of CDMA-HDR a High Efficiency-High Data Rate Personal Communication Wireless System” written by A. Jalali et al., IEEE 51st VTC2000-Spring, Volume 3, pages 1854 to 1858, May 2000, there has been reported the improvement in the frequency utilization efficiency of the cells by the application of the packet scheduling system called “Proportional Fairness” to the EV-DO system.
As typical packet scheduling systems, the following three sorts of systems are proposed:
(1) Maximum CIR system;
(2) Round Robin system; and
(3) Proportional Fairness system.
In the above-mentioned (1) Maximum CIR system, a transmission chance is allocated with a priority with respect to a mobile station having a superior communication quality of a downlink. Since a communication chance with respect to such a mobile station located in the vicinity of a base station is increased and a communication chance with respect to a remote mobile station is decreased, the Maximum CIR system corresponds to such a scheduling system that a service gap among the mobile stations becomes large. In the above-mentioned (2) Round Robin system, communication chances are uniformly allocated to all of the mobile stations. In the Round Robin system, the throughput of the base station is lowered, because the communication chances with respect to the remote mobile stations are increased, as compared with the throughput of the Maximum CIR system. In the above-mentioned (3) Proportional Fairness system, while instantaneous downlink communication quality/averaged downlink communication quality are employed as evaluation values, transmission chances are allocated to mobile stations having larger evaluation values with priorities. As a consequence, in the Proportional Fairness system, the communication chances are uniformly allocated, and further, the frequency utilization efficiency thereof becomes higher than that of the Round Robin system. It should be noted that the base station is required to correctly grasp instantaneous downlink communication qualities of the respective mobile stations.
In the EV-DO system, a mobile station estimates a downlink communication quality (SINR: Signal to Interference plus Noise Ratio) from a reception pilot signal, designates both a modulation system and a coding ratio (MCS: Modulation and Coding Scheme) that enable communication with a packet error lower than or equal to 1% in the SINR with reference to a table, and feeds back an index (DRC: Data Rate Control) corresponding to both the modulation system and the coding ratio to a base station. With employment of the above-described mechanism, the base station can grasp the instantaneous communication quality with respect to each of the mobile stations.
Since this operation may constitute the adaptive modulation for selecting the MCS in response to the feedback DRC, when the mobile station can correctly estimate the downlink communication quality, then the wireless communication can be carried out in higher efficiencies which are nearly equal to limitations in communication capacities. It should be noted that the packet scheduling is carried out after the instantaneous downlink communication quality has been fed back. As a result, an error may be produced when the SINR is estimated and when the SINR is acquired after completion of the packet scheduling due to a temporal changing factor such as a fading phenomenon.